Reductive dechlorination of polychlorinated biphenyls: threshold concentration and dechlorination kinetics of individual congeners in Aroclor 1248

Short-chain fatty acids facilitated long-term dechlorination of PCBs in Taihu Lake sediment microcosms: Evidence from PCB congener and microbial community analyses

Microbial reductive dechlorination hosts great promise as an in situ bioremediation strategy for polychlorinated biphenyls (PCBs) contamination. However, the slow dechlorination in sediments limits natural attenuation. Short-chain fatty acids, as preferred carbon sources and electron donors for dechlorinating microorganisms, might stimulate PCB dechlorination. Herein, two sets of short-chain fatty acids, sole acetate and a fatty acid mixture (acetate, propionate, and butyrate), were amended periodically into Taihu Lake (China) sediment microcosms containing nine PCB congeners (PCB5, 12, 64, 71, 105, 114, 149, 153, and 170) after 24 weeks of incubation. Short-chain fatty acids facilitated the long-term PCB dechlorination and the promoting effect of the fatty acid mixture compared favorably with that of sole acetate. By the end of 108 weeks, the total PCB mass concentrations in acetate amended and fatty acid mixture amended microcosms significantly declined by 7.6% and 10.3% compared with non-amended microcosms (P < 0.05), respectively. Short-chain fatty acids selectively favored the removal of flanked meta and single-flanked para chlorines. Notably, a rare ortho dechlorination pathway, PCB25 (24-3-CB) to PCB13 (3-4-CB), was enhanced. Supplementary fatty acids significantly increased reductive dehalogenases (RDase) gene pcbA5 instead of improving the growth of Dehalococcoides. These findings highlight the merits of low cost short-chain fatty acids on in situ biostimulation in treating PCBs contamination.

Characterization of mono- to deca-chlorinated biphenyls in a well-preserved sediment core from Beppu Bay, Southwestern Japan: Historical profiles, emission sources, and inventory

Contamination levels and profiles of mono- to deca-chlorinated biphenyls (PCBs) were characterized in a sediment core dated in 1954-2011 from Beppu Bay, southwestern Japan, providing a comprehensive and detailed picture on the environmental occurrence, temporal trends, and emission sources of these pollutants in the study area. Concentrations of total PCBs in the core ranged from 3.5 to 150 (median 15) ng g^-1 dry weight and exhibited depth profile matching with Japanese PCB production and emission patterns (i.e., drastically increasing from the early 1960s, peaking in 1970, and then rapidly decreasing). Origin of PCBs in the studied samples largely associated with Kanechlor mixtures (e.g., KC-300 and KC-400), especially for sediment layers dated between the mid-1960s and early 1970s (i.e., the intensive PCB production period in Japan). In addition, dechlorination and weathering signals and emerging inputs of PCBs were also observed in deeper and shallower sediment segments with notable proportions of some unique congeners such as CB-47/48/51 and CB-11, respectively. Historical fluxes of PCBs in our samples showed quite similar vertical shape as concentrations. In the context of national implementation for complete treatment of PCB-containing waste until 2024, further investigations on spatiotemporal trends and environmental loads of PCBs in Japan are necessary.

Bioremediation of typical chlorinated hydrocarbons by microbial reductive dechlorination and its key players: A review

Chlorinated hydrocarbon contamination in soils and groundwater has a severe negative impact on the human health. Microbial reductive dechlorination is a major degradation pathway of chlorinated hydrocarbon in anaerobic subsurface environments, has been extensively studied. Recent progress on the diversity of the reductive dechlorinators and the key enzymes of chlororespiration has been well reviewed. Here, we present a thorough overview of the studies related to bioremediation of chloroethenes and polychlorinated biphenyls based on enhanced in situ reductive dechlorination. The major part of this review is to provide an up-to-date summary of functional microorganisms which are either detected during in situ biostimulation or applied in bioaugmentation strategies. The applied biostimulants and corresponding reductive dechlorination products are also summarized and the future research needs are finally discussed.

Diverse dechlorinators and dechlorination genes enriched through amendment of chlorinated natural organic matter fractions

In uncontaminated environments, chlorinated natural organic matter (Cl-NOM) can act as an electron acceptor for organohalide-respiring bacteria. It is unknown, however, whether different types of Cl-NOM are preferentially dechlorinated or whether enrichment with Cl-NOM affects the ability of bacteria to dechlorinate contaminants. In this research NOM was extracted from sediment, fractionated based on hydrophobicity, and either amended to polychlorinated biphenyl-contaminated soil directly or chlorinated and then amended to soil. Amendments of the least hydrophobic Cl-NOM fraction were dechlorinated most rapidly, followed by the moderately hydrophobic Cl-NOM fraction. Soil that had been enriched on the moderately hydrophobic fraction of Cl-NOM was also capable of faster dechlorination of the contaminants trichloroethene and tetrachlorobenzene. Community analysis of the soil during enrichment showed that some known organohalide-respiring bacteria were present and may have played a role in dechlorination; nevertheless, many bacteria appeared to be enriched during both Cl-NOM and contaminant dechlorination. In addition, the quantities of two haloalkane dehalogenase genes increased during enrichment on Cl-NOM. These results show for the first time that Cl-NOM can prime contaminant dechlorination and also suggest that hydrolytic dechlorination processes were involved in both Cl-NOM and contaminant dechlorination.

Kinetics of PCB Microbial Dechlorination Explained by Freely Dissolved Concentration in Sediment Microcosms

While microbial dechlorination of polychlorinated biphenyls (PCBs) has been observed in sediments over the last 3 decades, translation to the field has been difficult due to a lack of a clear understanding of the kinetic limitations. To address this issue, the present study used passive dosing/sampling to accurately measure the biological rate of dechlorination of 2,3,4,5-tetrachlorobiphenyl (PCB 61) to 2,3,5-trichlorobiphenyl (PCB 23) by an organohalide-respiring bacterium, Dehalobium chlorocoercia (DF-1). The biological rates were measured over an environmentally relevant concentration range of 1-50 ng/L of freely dissolved concentrations with and without the presence of sediment in bench-scale microcosm studies. The rate of dechlorination was found to be linearly dependent on the freely dissolved concentration of PCB 61 both in sediment and in sediment-free microcosms. The observed rate of dechlorination in sediment microcosms could be predicted within a factor of 2 based on the kinetics measured in sediment-free microcosms. A threshold for dechlorination was not observed down to an aqueous concentration of about 1 ng/L PCB 61. We demonstrate that with the combination of an accurate measurement of the aqueous-phase dechlorination kinetics and an understanding of the site-specific partitioning characteristics, it is possible to predict PCB microbial dechlorination in sediments.

Estimation of rate constants of PCB dechlorination reactions using an anaerobic dehalogenation model

This study aims to estimate anaerobic dechlorination rate constants (k_m) of reactions of individual PCB congeners using data from four laboratory microcosms set up using sediment from Baltimore Harbor. Pathway k_m values are estimated by modifying a previously developed model as Anaerobic Dehalogenation Model (ADM) which can be applied to any halogenated hydrophobic organic (HOC). Improvements such as handling multiple dechlorination activities (DAs) and co-elution of congeners, incorporating constraints, using new goodness of fit evaluation led to an increase in accuracy, speed and flexibility of ADM. DAs published in the literature in terms of chlorine substitutions as well as specific microorganisms and their combinations are used for identification of pathways. The best fit explaining the congener pattern changes was found for pathways of Phylotype DEH10, which has the ability to remove doubly flanked chlorines in meta and para positions, para flanked chlorines in meta position. The range of estimated k_m values is between 0.0001-0.133d^-1, the median of which is found to be comparable to the few available published biologically confirmed rate constants. Compound specific modelling studies such as that performed by ADM can enable monitoring and prediction of concentration changes as well as toxicity during bioremediation.

DFT Studies of SN2 Dechlorination of Polychlorinated Biphenyls

Nucleophilic dechlorination of all 209 PCBs congeners by ethylene glycol anion has been studied theoretically at the DFT level. The obtained Gibbs free energies of activation are in the range 7-22 kcal/mol. The reaction Gibbs free energies indicate that all reactions are virtually irreversible. Due to geometric constrains these reactions undergo rather untypical attack with attacking oxygen atom being nearly perpendicular to the attacked C-Cl bond. The most prone to substitution are chlorine atoms that occupy ortho- (2, 2', 6, 6') positions. These results provide extensive information on the PEG/KOH dependent PCBs degradation. They can also be used in further developments of reaction class transition state theory (RC-TST) for description of complex reactive systems encountered for example in combustion processes.

Impacts of PCB analytical interpretation uncertainties on dechlorination assessment and remedial decisions

Laboratory analyses of polychlorinated biphenyls (PCBs) often do not quantitate the 209 individual PCB congeners, thereby requiring analyst interpretation to determine individual congener concentrations. Error introduced during this interpretation is subsequently propagated to calculated surrogate variables, such as the number of chlorines per biphenyl (CPB), and the molar dechlorination product ratio (MDPR), which are used to assess the extent of dechlorination and inform remedial decisions. The present work applies a Monte Carlo (MC) analysis to assess current methods for quantitating co-eluting congeners and the errors that could occur in individual congeners and derived CPB and MDPR estimates. Synthetic chromatograms, which were created using two alternative methods (random assignment and assignment based on relative proportions in Aroclors) for assigning mass to co-eluting congeners, were compared to their fully-quantitated counterparts. The percent error introduced in total PCB (∑PCB) concentration ranges from approximately -60% to +50%. Similarly, the errors associated with CPB and MDPR estimates range from approximately -20% to +20% and -120% to +30%, respectively. Uncertainties introduced during congener analysis and propagated to surrogate variables can thus be substantial, and should be considered in assessments of the extent of dechlorination and associated remedial decisions.

Microbial dechlorination of polychlorinated biphenyls, dibenzo-p-dioxins, and -furans at the Portland Harbor Superfund site, Oregon, USA

The Portland Harbor (Oregon, USA) has been declared a "Superfund" site because it is impacted by a variety of contaminants, including polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs). Using data collected in the remedial investigation, concentrations of PCBs and PCDD/Fs in sediment and water were examined using positive matrix factorization to look for evidence that PCBs and PCDD/Fs are dechlorinated by anaerobic bacteria. This process has long been known to occur in sediments. Recently, it has been recognized that PCB and PCDD/F dechlorination may also occur in other anaerobic environments, such as in landfills, sewers, and groundwater. The results indicate that a factor related to the dechlorination of PCBs and PCDD/Fs was present in the water but not in the sediment. Spatial patterns in dechlorination products suggest that they come primarily from groundwater. Dechlorination products comprise 22% of the PCBs in the water. The Portland Harbor therefore represents the third major US watershed in which PCBs appear to undergo dechlorination in an environment other than sediment, suggesting that the microbial dechlorination of PCBs and PCDD/Fs is more common than previously assumed. In addition, the Portland Harbor is impacted by PCBs generated inadvertently during the production of pigments, such as PCB 11, which alone exceeded the 64 pg/L federal water quality standard for the sum of PCBs in two of 120 whole water samples.

Evidence for photochemical and microbial debromination of polybrominated diphenyl ether flame retardants in San Francisco Bay sediment

Brominated diphenyl ethers (BDEs) are flame retardant compounds that have been classified as persistent organic pollutants under the Stockholm Convention and targeted for phase-out. Despite their classification as persistent, PBDEs undergo debromination in the environment, via both microbial and photochemical pathways. We examined concentrations of 24 PBDE congeners in 233 sediment samples from San Francisco Bay using Positive Matrix Factorization (PMF). PMF analysis revealed five factors, two of which contained high proportions of congeners with two or three bromines, indicating that they are related to debromination processes. One of the factors included PBDE 15 (4,4'-dibromo diphenyl ether, comprising 20% of the factor); the other included PBDE 7 (2,4-dibromo diphenyl ether; 12%) and PBDE 17 (2,2',4-tribromo diphenyl ether; 16%). The debromination processes that produce these congeners are probably photochemical debromination and anaerobic microbial debromination, although other processes could also be responsible. Together, these two debromination factors represent about 8% of the mass and 13% of the moles of PBDEs in the data matrix, suggesting that PBDEs undergo measurable degradation in the environment.

Kinetics and threshold level of 2,3,4,5-tetrachlorobiphenyl dechlorination by an organohalide respiring bacterium

The time required for a PCB-contaminated site to recover cannot yet be predicted due in part to lack of quantitative information on rates of PCB dechlorination in the porewater phase. We developed a method to measure rate of dechlorination in the aqueous phase at very low PCB concentrations. This approach utilizes a polymer functioning concurrently as a passive dosing system for maintaining a steady-state PCB substrate concentration in the water phase and as a passive equilibrium sampler to monitor the dechlorination product. Rates of dechlorination of 2,3,4,5-tetrachlorobiphenyl (PCB 61) to 2,3,5-trichlorobiphenyl (PCB 23) by an organohalide respiring bacterium, Dehalobium chlorocoercia DF-1, were measured over an environmentally relevant range of 1 to 500 ng L(-1) in sediment-free medium using a high concentration of cells (>10(6) cells mL(-1)). The results indicate that rate of dechlorination is a linear function of PCB substrate concentration below the maximum aqueous solubility of PCB 61 and occurs at concentrations as low as 1 ng L(-1). Demonstration of PCB 61 dechlorination at environmentally relevant concentrations suggests that low numbers of organohalide respiring bacteria rather than bioavailability accounts for low rates of dechlorination typically observed in sediments. Using passive samplers to measure the concentration of dissolved PCBs in the porewater combined with knowledge of congener-specific rates for organohalide respirer(s), it will be possible to project the in situ rate and final concentration of PCBs for a specific site after treatment by bioaugmentation.

Effects of activated carbon on reductive dechlorination of PCBs by organohalide respiring bacteria indigenous to sediments

Polychlorinated biphenyls (PCBs) have accumulated in aquatic sediments due to their inherent chemical stability and their presence poses a risk due to their potential toxicity in humans and animals. Granular activated carbon (GAC) has been applied to PCB contaminated sediment sites to reduce the aqueous concentration by sequestration thus reducing the PCB exposure and toxicity to both benthic and aquatic organisms. However, it is not known how the reduction of PCB bioavailability by adsorption to GAC affects bacterial transformation of PCBs by indigenous organohalide respiring bacteria. In this study, the impact of GAC on anaerobic dechlorination by putative organohalide respiring bacteria indigenous to sediment from Baltimore Harbor was examined. It was shown that the average Cl/biphenyl after dehalogenation of Aroclor 1260 was similar between treatments with and without GAC amendment. However, GAC caused a substantial shift in the congener distribution whereby a smaller fraction of highly chlorinated congeners was more extensively dechlorinated to mono- through tri-chlorinated congeners compared to the formation of tri- through penta-chlorinated congeners in unamended sediment. The results combined with comparative sequence analysis of 16S rRNA gene sequences suggest that GAC caused a community shift to putative organohalide respiring phylotypes that coincided with more extensive dechlorination of ortho and unflanked chlorines. This shift in activity by GAC shown here for the first time has the potential to promote greater degradation in situ by promoting accumulation of less chlorinated congeners that are generally more susceptible to complete mineralization by aerobic PCB degrading bacteria.

Overview of in situ and ex situ remediation technologies for PCB-contaminated soils and sediments and obstacles for full-scale application

Polychlorinated biphenyls (PCB) are persistent organic pollutants used worldwide between the 1930s and 1980s. Although their use has been heavily restricted, PCB can be found in contaminated soils and sediments. The most frequent remediation solutions adopted are "dig and dump" and "dig and incinerate", but there are currently new methods that could be more sustainable alternatives. This paper takes a look into the remediation options available for PCB-contaminated soils and sediments, differentiating between biological, chemical, physical and thermal methods. The use of combined technologies was also reviewed. Most of them are still in an initial development stage and further research in different implementation issues is needed. There is no single technology that is the solution for PCB contamination problem. The successful remediation of a site will depend on proper selection, design and adjustment of the technology or combined technologies to the site characteristics.

Review on the fate of emerging contaminants during sludge anaerobic digestion

Several research papers have been published during the last years investigating the occurrence, fate and effects of emerging contaminants (ECs) on sludge anaerobic digestion (AD). Literature review revealed that research has been mainly focused on specific groups of compounds (linear alkylbenzene sulphonates, nonylphenol ethoxylates, some pharmaceuticals, estrogens, phthalates), while there are fewer or no data for others (personal care products, perfluorinated compounds, brominated flame retardants, organotins, benzotriazoles, benzothiazoles, nanoparticles). AD operational parameters (sludge residence time, temperature), sludge characteristics (type of sludge, adaptation on the compound), physicochemical properties of ECs and co-metabolic phenomena seem to affect compounds' biodegradation. The use of sludge pretreatment methods does not seem to enhance ECs removal; whereas encouraging results have been reported when AD was combined with other treatment methods. Future efforts should be focused on better understanding of biotransformation processes and sorption phenomena occurred in anaerobic digesters, as well as on identification of (bio)transformation products.

Aerobic degradation of 3-chlorobenzoic acid by an indigenous strain isolated from a polluted river

An indigenous strain of Pseudomonas putida capable of degrading 3-chlorobenzoic acid as the sole carbon source was isolated from the Riachuelo, a polluted river in Buenos Aires. Aerobic biodegradation assays were performed using a 2-l microfermentor. Biodegradation was evaluated by spectrophotometry, chloride release, gas chromatography and microbial growth. Detoxification was evaluated by using Vibrio fischeri, Pseudokirchneriella subcapitata and Lactuca sativa as test organisms. The indigenous bacterial strain degrades 100 mg l(-1) 3-chlorobenzoic acid in 14 h with a removal efficiency of 92.0 and 86.1% expressed as compound and chemical oxygen demand removal, respectively. The strain was capable of degrading up to 1,000 mg of the compound l(-1). Toxicity was not detected at the end of the biodegradation process. Besides initial concentration, the effect of different factors, such as initial pH, initial inoculum, adaptation to the compound and presence of other substrates and toxic related compounds, was studied.

The effects of individual PCB congeners on the soil bacterial community structure and the abundance of biphenyl dioxygenase genes

Polychlorinated biphenyls (PCBs) are toxic environmental contaminants that represent a class of 209 congeners characterized by different degrees of chlorination and substitution patterns. Most of experimental studies about microbial degradation of PCBs have been conducted on PCB mixtures, even though evidence accumulated in bacteria and other organisms shows that exposure to different congeners may have different biological effects. Microcosm experiments were conducted using aerobic agitated soil slurries individually exposed to PCB congeners with different degrees of chlorination: PCB-3, 15, 28, and 77, and the commercial mixture Aroclor 1242. After four weeks of incubation, PCBs were analyzed by gas chromatography/mass spectrometry (GC/MS) showing different transformation extents: With the exception of PCB-15 that was not significantly transformed (7%), biodegradation rates decreased with the degree of chlorination, from 75% for PCB-3 to 22% for PCB-77 and Aroclor 1242. The bacterial abundance, as measured by colony counting and 16S rDNA quantification by real-time PCR, was lower (of about 40%) in soil microcosms exposed to the higher-chlorinated congeners, PCB-28, PCB-77, and Aroclor 1242, as compared to non-exposed soils and soils exposed to the lower-chlorinated congeners, PCB-3 and PCB-15. The relative abundance of different taxonomic groups, as determined by real-time PCR, revealed an increase of β-Proteobacteria and Actinobacteria in all microcosms exposed to PCBs, as compared with non-exposed soil. In addition, exposure to PCB-77 and Aroclor 1242 resulted in a higher abundance of α-Proteobacteria and Acidobacteria. Globally, these results suggest that exposure to PCBs (and especially to higher-chlorinated congeners and Aroclor 1242) selected bacterial groups involving most known PCB degraders, i.e., β-Proteobacteria and Acidobacteria. The quantification of biphenyl dioxygenase (BPH) genes--involved in the aerobic degradation of PCBs--using real-time PCR showed that exposure to all PCB congeners and Aroclor 1242 resulted in a marked increase of two out of the four BPH genes tested, similarly suggesting the selection of PCB-degrading bacteria. This paper showed that exposure to different PCB congeners leads to different structures of the soil bacterial community and BPH genes expression patterns.

Evidence for widespread dechlorination of polychlorinated biphenyls in groundwater, landfills, and wastewater collection systems

One of the few pathways for environmental transformation of polychlorinated biphenyls (PCBs) is microbial dechlorination under anaerobic conditions, which is reported to occur in contaminated sediments of rivers, lakes and harbors. The goal of this work was to determine whether PCB dechlorination occurs in built waste treatment environments. We analyzed a large database on PCB congener concentrations in effluents and some influents of facilities in the Delaware River Basin. Positive matrix factorization was used to identify the sources of PCBs and to look for evidence of dechlorination. Seven factors were resolved from the data set of 89 congeners in 645 samples. Two of the resolved factors represented dechlorination signals. One of these was dominated by PCBs 4 and 19 and represents an advanced stage of dechlorination of Aroclors to di- and trichlorinated congeners. This dechlorination signal was most prevalent in effluents from sites with contaminated groundwater and from wastewater treatment plants (WWTPs) that serve combined sewers or treat landfill leachate. The other dechlorination signal appeared to represent an intermediate stage of dechlorination, because it was dominated by two coeluting groups of tetrachlorinated congeners: PCBs 44 + 47 + 65 and 45 + 51. This partial dechlorination signal was most prevalent in the 40 WWTPs with separate (sanitary) sewer systems, where it often comprised more than 20% of the PCBs in the effluents. Both dechlorination signals were present in WWTP influents, but were not observed in stormwater runoff, suggesting that dechlorination occurs in sewers. This work represents the first convincing evidence of PCB dechlorination occurring outside of contaminated aquatic sediments or anaerobic digesters. The results suggest that PCBs are dechlorinated by anaerobic bacteria in sewers, landfills, and contaminated groundwater. These two dechlorination signals comprise about 19% of the total loads of PCBs to the Delaware River from the sampled dischargers.

Identification of structural properties associated with polychlorinated biphenyl dechlorination processes

Polychlorinated biphenyl molecules can be biologically dechlorinated through sequential losses of a chlorine atom, following 840 pathways from higher chlorinated to lesser-chlorinated congeners and biphenyl. Previously, eight recurring sets of pathways, herein referred to as explicitly reported pathways in dechlorination processes, have been identified through qualitative analysis of shifts in congener masses in field and laboratory studies. Dechlorination process generalizations were qualitatively extrapolated based on limited attributes of the congeners dechlorinated in the explicitly reported pathways. They are valuable because they allow comparisons of dechlorination patterns across laboratory experiments and contaminated sites. However, due to analytical limitations and a paucity of studies, the explicitly reported pathways in dechlorination processes likely do not represent all of the pathways that could occur at contaminated sites. This work presents an alternative, quantitative, and replicable approach to the identification of candidate pathways for inclusion in dechlorination process generalizations through use of classification trees. This method considers 46 structural and property attributes of dechlorination pathways. Trees fit for pathway inclusion in each of the eight dechlorination processes with alternative assumptions are compared in terms of critical congener attributes. The classification trees correctly classify explicitly reported pathways into dechlorination processes at rates of 0.90 to 0.99. While many of the attributes used in the original generalizations were also selected as predictors by the classification trees, the extra attributes allow identification of additional dechlorination pathways that can be considered as candidates for monitoring in future studies.

Phytoremediation of polychlorinated biphenyls: new trends and promises

Transgenic plants and associated bacteria constitute a new generation of genetically modified organisms for efficient and environment-friendly treatment of soil and water contaminated with polychlorinated biphenyls (PCBs). This review focuses on recent advances in phytoremediation for the treatment of PCBs, including the development of transgenic plants and associated bacteria. Phytoremediation, or the use of higher plants for rehabilitation of soil and groundwater, is a promising strategy for cost-effective treatment of sites contaminated by toxic compounds, including PCBs. Plants can help mitigate environmental pollution by PCBs through a range of mechanisms: besides uptake from soil (phytoextraction), plants are capable of enzymatic transformation of PCBs (phytotransformation); by releasing a variety of secondary metabolites, plants also enhance the microbial activity in the root zone, improving biodegradation of PCBs (rhizoremediation). However, because of their hydrophobicity and chemical stability, PCBs are only slowly taken up and degraded by plants and associated bacteria, resulting in incomplete treatment and potential release of toxic metabolites into the environment. Moreover, naturally occurring plant-associated bacteria may not possess the enzymatic machinery necessary for PCB degradation. To overcome these limitations, bacterial genes involved in the metabolism of PCBs, such as biphenyl dioxygenases, have been introduced into higher plants, following a strategy similar to the development of transgenic crops. Similarly, bacteria have been genetically modified that exhibit improved biodegradation capabilities and are able to maintain stable relationships with plants. Transgenic plants and associated bacteria bring hope for a broader and more efficient application of phytoremediation for the treatment of PCBs.

Dynamics of selected pre-existing polybrominated diphenylethers (PBDEs) in municipal wastewater sludge under anaerobic conditions

Despite the rapid, widespread accumulation of polybrominated diphenylethers (PBDEs) in our surroundings, their environmental fate has been largely unknown. In the present study, most common congeners (BDE 47, 99, 100, 138, 153, 154, 183 and 209) were investigated for their dynamics in municipal sewage sludge under mesophilic condition (37 degrees C). In anaerobic batch cultures, the concentrations of BDE 47, 99, 100 and 209, exhibited significant decreases (by 22-40% from their initial concentration), whereas the levels of the other congeners, BDE 138, 153, 154 and 183, remained stable during a 238-d incubation. However, in a parallel study conducted in a pilot-scale anaerobic sludge digester, loss of all eight congeners was observed. The present study indicates that certain PBDE congeners undergo a significant mass decrease under anaerobic conditions.

PCB dechlorination enhancement in Anacostia River sediment microcosms

In situ treatment of PCB contaminated sediments via microbial dechlorination is a promising alternative to dredging, which may be reserved for only the most contaminated areas. Reductive dechlorination of low levels of weathered PCB mixtures typical of urban environments may occur at slow rates. Here, we report that biostimulation and bioaugmentation enhanced dechlorination of low concentration (2.1 mg PCBs/kg dry weight) historical PCBs in microcosms prepared with Anacostia River, Washington, DC, sediment. Treatments included electron donors butyrate, lactate, propionate and acetate (1 mM each); alternate halogenated electron acceptors (haloprimers) tetrachlorobenzene (TeCB, 25 microM), pentachloronitrobenzene (PCNB, 25 microM), or 2,3,4,5,6-PCB (PCB116, 2.0 microM); and/or bioaugmentation with a culture containing Dehalococcoides ethenogenes strain 195 (3 x 10(6)cells/mL). Dechlorination rates were enhanced in microcosms receiving bioaugmentation, PCNB and PCNB plus bioaugmentation, compared to other treatments. Microcosm subcultures generated after 415 days and spiked with PCB116 showed sustained capacity for dechlorination of PCB116 in PCNB, PCNB plus bioaugmentation, and TeCB treatments, relative to other treatments. Analysis of Chloroflexi 16S rRNA genes showed that TeCB and PCNB increased native Dehalococcoides spp. from the Pinellas subgroup; however this increase was correlated to enhanced dechlorination of low concentration weathered PCBs only in PCNB-amended microcosms. D. ethenogenes strain 195 was detected only in bioaugmented microcosms and decreased over 281 days. Bioaugmentation with D. ethenogenes strain 195 increased PCB dechlorination rates initially, but enhanced capacity for dechlorination of a model congener, PCB116, after 415 days occurred only in microcosms with enhanced native Dehalococcoides spp.

The reductive dechlorination of 2,3,4,5-tetrachlorobiphenyl in three different sediment cultures: evidence for the involvement of phylogenetically similar Dehalococcoides-like bacterial populations

Anaerobic cultures capable of reductively dechlorinating 2,3,4,5-tetrachlorobiphenyl (CB) were enriched from three different sediments, one estuarine, one marine and one riverine. Two different electron donors were used in enrichments with the estuarine sediment (elemental iron or a mixture of fatty acids). The removal of doubly flanked meta and para chlorines to form 2,3,5-CB and 2,4,5-CB was observed in all cultures. Bacterial community analysis of PCR-amplified 16S rRNA gene fragments revealed different communities in these cultures, with the exception of one common population that showed a high phylogentic relatedness to Dehalococcoides species. No Dehalococcoides-like populations were ever detected in control cultures to which no PCBs were added. In addition, the dynamics of this Dehalococcoides-like population were strongly correlated with dechlorination. Subcultures of the estuarine sediment culture demonstrated that the Dehalococcoides-like population disappeared when dechlorination was inhibited with 2-bromoethanesulfonate or when 2,3,4,5-CB had been consumed. These results provide evidence that Dehalococcoides-like populations were involved in the removal of doubly flanked chlorines from 2,3,4,5-CB. Furthermore, the successful enrichment of these populations from geographically distant and geochemically distinct environments indicates the widespread presence of these PCB-dechlorinating, Dehalococcoides-like organisms.